A Study on Photodegradation and The Fate of Pyrogenic Carbon

<p>Accounting for all possible sources of atmospheric CO₂ is a pressing issue today due to the increasing effects of climate change. Estimates suggested that on the order of 1.3 million tons of dissolved pyrogenic carbon (<em>py</em>DOC) could be entering the northern Gulf of Mexico annually. Assessing the fate of <em>py</em>DOC in aquatic systems is crucial to understanding ecosystem impacts and potential feedback to climate change. Current research indicates that despite a generally lower susceptibility to biodegradation than their unpyrolyzed equivalents, pyrogenic carbon is not environmentally inert. While the role of microorganisms on the fate and transformation of dissolved pyrogenic carbon is well understood, very little research has been conducted to quantify contributions of abiotic processes such as photodegradation. The purpose of this study was to assess photodegradation of <em>py</em>DOC and quantify its byproducts (specifically, CO₂ evolved). The study consisted of a complimentary mix of laboratory-scale and controlled field-scale experiments with objectives of: 1) To assess the influence of solar irradiance on photodegradation of dissolved pyrogenic carbon and the nature of the degraded byproducts, 2) To assess spatiotemporal variation in photodegradation of <em>py</em>DOC in a freshwater and saltwater system, and 3) To assess the impact of salinity and depth below the water on photodegradation rates.</p> <p>Natural charcoal was collected from a prescribed burn site, dissolved, and diluted to create a stock solution for testing. A sampling apparatus was made to hold the samples in a stable position at the water surface. Samples were exposed to natural light with repeated experiments on days of varying irradiance (cloudiness). Variations in spatiotemporal influences were assessed by placing samples at different distances from shore both in a freshwater lake and in the Mississippi Sound. Experiments took place over three days with samples being collected every four hours. Following exposure, samples were tested using UV visible spectrophotometry, gas chromatography, as well as pH and electrical conductivity. Results indicated that photodegradation accounts for a 50% loss of dissolved pyrogenic carbon with the amount of photodegradation being proportional to solar irradiance. As a byproduct of photodegradation, the samples exposed to light were found to have on average more than twice as much headspace CO₂ as the dark control samples. This could be a sizable unknown source for atmospheric CO₂. Side by side tests of samples in a saline solution and samples in a distilled water solution were exposed and were found to degrade at similar rates and produce equal quantities of CO₂<a>[CB1]</a> . Samples were also exposed at different depths below the surface of the water at 3, 6<a>[CB2]</a> , and 12 inches. Samples at 3 inches lost 14%, while samples at 12 inches lost only 6% after one day of exposure with a total of 6.2 kW h/m². A curve was then constructed to predict the loss of dissolved pyrogenic carbon at a certain depth with 6.2 kW h/m² in one day.</p>

Name thesis:degree_name

Master of Science (MS)

Level thesis:degree_level

Masters Thesis

Discipline thesis:degree_discipline

Geography and Geology

Year dc:date.available

2015

Author dc:creator

• Howell, John Thomas

Contributors dc:contributor

• Omar Harvey
• Franklin Heitmuller
• Alan Shiller

• pyrogenic
• carbon
• photodegradation
• Geology

Repository record dc:identifier

https://aquila.usm.edu/masters_theses/89

OAI identifier oai:identifier

oai:aquila.usm.edu:masters_theses-1086